1. Field of the Invention
The invention relates to a control apparatus and a control method of a continuously variable transmission, a program for realizing that method, and a recording medium on which that program is recorded. More specifically, the invention relates to technology for controlling the squeezing pressure applied to a transmission belt of a continuously variable transmission.
2. Description of the Related Art
A continuously variable transmission (hereinafter simply referred to as “CVT”) is known in which a transmission belt is wound around a pair of pulleys, the gap widths of which can be changed. In this kind of continuously variable transmission, squeezing pressure is applied to the transmission belt so that it will not slip. However, during sudden braking of the vehicle, for example, the output rotation speed of the CVT rapidly decreases. As a result, excessive torque is transmitted to the transmission belt which might cause it to slip. Therefore, in order to prevent the transmission belt from slipping during sudden braking, there is technology which increases the squeezing pressure that is applied to the transmission belt during sudden braking.
Japanese Patent Application Publication No. 8-210450 (JP-A-8-210450) descries a line pressure control apparatus that can appropriately control line pressure in a V-belt type continuously variable transmission so that the line pressure is neither excessive nor insufficient when the vehicle is braked. The line pressure control apparatus described in JP-A-8-210450 includes a bake detecting portion, a speed ratio calculating portion, and a braking line pressure increasing portion. The brake detecting portion detects braking in a vehicle provided with a V-belt type continuously variable transmission structured such that line pressure is applied to a variable flange of one of two pulleys around which a V-belt is wound, shift control pressure obtained by reducing the line pressure using a shift control valve is applied to the variable flange of the other pulley, and a speed ratio is controlled in a stepless fashion according to the differential pressure between the shift control pressure and the line pressure. The speed ratio calculating portion calculates the actual speed ratio of the V-belt type continuously variable transmission. The braking line pressure increasing portion increases the line pressure to a value set according to the actual speed ratio during braking of the vehicle.
According to the line pressure control apparatus described in that publication, the line pressure is increased to a value that is set according to the actual speed ratio of the continuously variable transmission during braking of the vehicle. Accordingly, the line pressure can be controlled so that the V-belt will not slip even when the wheels rapidly decelerate when the vehicle is being braked. As a result, it is possible to prevent the durability of the V-belt from decreasing.
However, with the line pressure control apparatus according to JP-A-8-210450, the line pressure is increased during braking which means that the belt squeezing pressure is increased even during gradual braking as well. During gradual braking, however, excessive torque is not input to the belt so there is no need to increase the amount of squeezing pressure applied to the belt. Conversely, increasing the amount of squeezing pressure applied to the belt during gradual braking adversely effects fuel efficiency as well as the durability of the belt.